With increasing Internet traffic, there is an increasing need to increase communication capacity for optical communication. Technologies receiving attention as a routing function device for the optical communication include optical switches. Among the optical switches, a free-space optical switch for switching between paths of light in a free space is superior to other types from the viewpoint of high-density packaging or reduction in power consumption, and its technological development has recently advanced.
A basic configuration of the free-space optical switch will be described. Generally, the free-space optical switch is formed of several lenses and a light beam deflector element for changing a direction of travel of a light beam, which are provided in a free space between an input fiber and an output fiber. Typical optical switches include an optical crossconnect switch (OXC) formed of an input/output fiber array, a collimator lens array, and two sets of light beam deflector element arrays, and a wavelength selector switch (WSS) formed of an input/output fiber array, a collimator lens array, a lens group, a dispersion element, and a beam deflector element array.
The free-space optical switch can lay out a light beam at high density in a space but, on the other hand, does not have an optical waveguide structure formed of a fiber or the like and thus poses a problem of unintended leakage of light outside a target output port. In a case of an optical switch having a configuration including an arrangement of plural output ports, leakage of light at high power level into a proximity port or the like is a matter of technological development which needs particular care, due to the fact that such leakage causes crosstalk and hence leads to degradation in signal quality.
Functions of the optical switch include an optical attenuation function for controlling the light power of the output port. Although the optical attenuation function of the free-space optical switch may be achieved by an approach of individually mounting an optical attenuator in the following stage after each output port, this increases cost or size with an increasing number of ports. Therefore, a method using the light beam deflector element in the optical switch to implement the attenuation function is conceivable.
In a case where the light beam deflector element is used to implement the attenuation function, how the aforementioned crosstalk can be suppressed is a problem. Micro Electro Mechanical Systems (MEMS) or Liquid Crystal on Silicon Spatial Light Modulator (LCOS-SLM) is used as the light beam deflector element. Attenuation approaches include a method in which a phase is not returned with 2π as disclosed in Non-patent Literatures 1, 2, and a method in which a light beam is deflected at an angle intentionally offset from an optimum angle to the output port to control a light coupling rate, as disclosed in Patent Literature 1. However, this approach presents the problem of leakage of light to an adjacent output port causing crosstalk. Therefore, technical development in a novel attenuation approach is sought.